One known type of coupling comprises:
first and second end pieces suitable for fixing to the facing ends of first and second shafts, said end pieces having respective longitudinal axes which substantially intersect at a coupling center, and having respective first and second concave cylindrical bearing surfaces which are of circular symmetry about respective first and second oscillation axes which also substantially intersect at the coupling center and which are perpendicular to respective ones of the said longitudinal axes of the first and second end pieces, said cylindrical bearing surfaces being of the same diameter;
an intermediate piece having first and second convex cylindrical bearing surfaces which are of circular symmetry about two substantially intersecting perpendicular oscillation axes, said intermediate part being disposed between the said end pieces which are oriented relative to one another in such a manner that the first and second convex bearing surfaces of the intermediate piece co-operate with the first and second concave bearing surfaces of the first and second end pieces respectively, thereby simultaneously enabling driving torque to be transmitted between said shafts when rotating about their axes, enabling axial thrust to be transmitted between said shafts, and, by rocking about said first and second oscillation axes, enabling said shafts to oscillate relative to each other;
each of said end pieces being in the form of a solid having a front face facing the other end piece, which front face is in the form of a convex dihedral with the line of intersection of the dihedral half-planes constituting the oscillation axis of the end piece, the dihedral angle being equal to 180.degree. less twice the maximum angle of deflection between the axes of the two end pieces, said concave cylindrical bearing surface being hollowed out in the front face of the end piece such that the cross section of the bearing surface is an arc of a circle extending over said dihedral angle, and such that, when said intermediate piece is received in said hollow and any force is applied thereto in a direction which is perpendicular both to the longitudinal axis of the end piece and to the oscillation axis of the end piece, and in particular when any torque is transmitted thereby, a longitudinal reaction is created tending to urge the end piece away from the intermediate piece, the length of the cylindrical concave bearing surface parallel to the generator lines of the cylinder being equal to the length of the convex bearing surface of the intermediate piece so as to prevent the intermediate piece from moving in the hollow of the concave bearing surface in a direction parallel to the oscillation axis of the end piece; and
a retaining member for holding said end pieces together against said longitudinal reaction.
Such a device is described in British Patent publication No. 2,110,792 A (Tous).
In this known device, the intermediate piece is in the form of two semi-cylindrical portions each having two plane semi-circular faces of diameter greater than the axial length of said semi-cylindrical portions, a cylindrical face extending over an arc of 180.degree. and a rectangular plane face at right angles to said end faces. The axis of each semi-cylindrical portion lies in the rectangular plane face thereof. Said portions are crossed and interconnected by their plane faces being placed against each other. Since the axial length of these portions is less than their diameter, each rectangular face has a central square which is in contact with the other rectangular face and also has two end portions extending beyond two sides of the square, in other words, there is a concave right angle dihedral on either side of each portion. As a result, torque is transmitted through a portion of the intermediate piece (i.e. the square) which has an area and a second moment of area which are both small relative to the area and second moment of area of the adjacent parts. Thus, particularly when space is limited, this arrangement is unsatisfactory both for transmitting thrust and for transmitting torque.
It appears that this known device is poor at transmitting large values of axial thrust and torque, stands up badly to shock in the axial direction, and is expensive both to manufacture and to maintain because of difficulties in assembly and disassembly.
Preferred embodiments of the present invention avoid these drawbacks provided the relative oscillation between the shafts is limited to a relatively small angle, e.g. not more than 10.degree. on either side of a nominal in-line position. In particular, the invention can provide the following advantages:
high axial thrust,
high values of transmitted torque,
compact,
stands up very well to axial shocks,
simple to assemble, disassemble, maintain and adapt to various environmental conditions of temperature and chemicals;
simple to manufacture.